The Voltage controlled oscillator (“VCO”) plays an important role in the operation of a Phase Lock Loop (“PLL”). A typical PLL generates an output signal the phase of which is related to the phase of a reference signal (the input signal).
As is known in the art, the gain of a VCO (KVCO) is defined as the operational frequency range divided by the control voltage range, typically in the units of MHz/V. While having a large KVCO is generally desirable (for example, a large KVCO may allow the VCO to be used in a diverse variety of applications), if KVCO becomes too large then stability and/or noise performance of the VCO will degrade which reduces the effectiveness of the VCO. With modern VCO applications, the value of voltage source VDD is reduced which consequently reduces the operating range of the control voltage signal VC and therefore increases KVCO. Additionally, modern VCO applications require higher operational speed and data rate thereby increasing the operational frequency range of the VCO which, consequently, increases KVCO even further. The result is that KVCO becomes too large to maintain the necessary stability and/or noise performance requirements.
Therefore, there is a need to have a VCO with a larger operational frequency range will maintaining a low KVCO to maintain stability and/or noise performance of the VCO.